Data from prospective seroepidemiologic studies continue to support the hypothesis that swine influenza virus (Hsw1N1) is maintained in swine herds by an unusual method, not involving lungworms, perhaps by chronic or latent infections in individual swine. However, it appears that current human subtypes of influenza A virus (H3N2 and H1N1) have not become established in swine in Hawaii. Limited experimentation with swine influenza virus on transplacental transmission and neonatal infection in swine, including the detection of virus by organ culture, has failed to provide insight into possible biologic mechanisms that allow the virus to survive in apparently immune herds.